The present invention relates to testing memory cell arrays to locate soft (potentially defective) cells. The present invention provides a method for testing memory cell arrays to locate soft cells and provides an apparatus for implementing the test.
Memory cell arrays as used in semiconductor RAMS are fabricated using increasingly small geometries. During fabrication, cells may be produced in the array which are shorted or opened because of flaws in the structure or defects in the fabrication process. These cells may be readily detected and located in post-fabrication testing because of their characteristics which are quite distinct and readily identifiable compared to the desired cell characteristics, i.e., these cells never store information as desired.
More problematical are other cells in the array which, although not shorted or opened, will not possess the desired cell characterisitics. For example, these cells may have imbalanced threshold voltages between a pair of transistors in the cell. Defects such as these do not necessarily result in cell failure, although they are potentially defective (soft) and would probably result in failure at some time during use. However, such cells are not easily detected or located during standard testing. Because the exact weakness or defect in the cell is not easily defined and may in fact range widely, there is no established test for locating these defective cells. As a consequence, many arrays may be approved for sale and use while actually containing defective cells. No good test apparatus or method for locating such soft cells has come to light despite the persistence of this problem for several years.
Therefore, it is desirable to devise a method for testing memory cell arrays to detect and locate cells which are soft (potentially defective) before the memory is sold. Similarly, it is desirable to provide circuitry for implementing this testing without affecting the performance of the memory cell array and without unduly increasing the cost of the final cell array.
Accordingly, it is a principal object of the present invention to provide a method for locating soft cells in a memory cell array.
It is a related object of the present invention to provide apparatus for implementing such testing for individual memory cell arrays.
Further objects are to provide method and apparatus which are easy to operate and which adapt to standard test equipment.